Bend insensitive gradient index multi-mode light conducting fiber

ABSTRACT

The invention relates to a bend insensitive gradient index multimode light conducting fiber comprising a leakage mode dependent optical core diameter that is uniform over its length and numerical aperture that is uniform over its length, a core ( 1 ), an inner cladding ( 2 ), a refraction index trench ( 3 ) and an outer cladding ( 4 ), wherein the core ( 1 ) includes a core radius R 1 , an alpha-refraction index profile and a core refraction index difference dn 1  with respect to the outer cladding ( 4 ), wherein the refraction index trench ( 3 ) includes a refraction index trench radius R 3  and a trench refraction index difference dn 3  with respect to the outer cladding ( 4 ), wherein the outer cladding ( 4 ) includes an outer cladding radius R 4  and a refraction index between 1.40 and 1.55, wherein for a light wavelength of 850 nm and an overfilled launch (OFL), the optical core diameter for a fiber length in a range between 2 m and 300 m decreases by less than 5% and the numerical aperture decreases by less than 2.5% and the curvature related attenuation increase for two turns and a curvature radius of 7.5 mm is less than 0.2 db.

The invention relates to a bend insensitive gradient index multi-modelight conducting fiber according to the preamble of patent claim 1.

A standard gradient index multimode light conducting fiber subsequentlydesignated as GIMM-fiber includes a fiber core with a so-calledalpha-refraction index profile, wherein the refraction index differenceof the core relative to a surrounding cladding essentially a powerfunction of the radius. The core is made from doped silica and thecladding is preferably made from non-doped fused silica. Fibers of thistype have already been used for many years.

It is known that the so-called macro-bend losses of fibers of this typecan be reduced by inserting a so-called refraction index trench betweenthe center core and the surrounding cladding. The refraction indextrench is a portion in which the refraction index is lowered relative tothe core and also relative to the cladding. Fibers of this type are alsodesignated as bend insensitive multimode fibers with the abbreviationBIMMF. The macro-bend properties of such BIMMF with respect to anattenuation increase, the measured wavelength, the number of turns andthe bend radius is described in standardized internationalspecifications.

It is a consequence of the trench concept to reduce the macro-bendlosses that in a BIMM light conducting fiber so-called leakage modes arealso generated as a function of excitation conditions besides thetypically occurring propagation capable modes. One of these frequentlyoccurring excitation types is designated as overfilled launch excitation(OFL excitation). For an excitation of this type, the directly lightconducting core of the light conductor fiber is overfilled throughexcitation with a light source. In OFL excitations, leakage modes aregenerated which can propagate over several hundred meters.

In standard GIMM light conducting fibers without the recited trenchconcept leakage modes can only be observed over a length of a fewmillimeters from the coupling location and they do not have anypractical significance. In BIMMF with trench structure, the leakagemodes, however, are run through the refraction index trench over alonger distance. This means that the increase in insensitivity relativeto macro-bends which is obtained through the trench concept of the BIMMFover the standard GIMM light conducting fibers causes a disadvantagethrough leakage mode conduction, in particular for an OFL excitation.

This leakage mode conduction has negative impacts on the opticalproperties of the light conducting fibers which depend from the fiberlength. GIMM light conducting fibers are defined with respect to theirlight conducting properties through the parameter of the optical corediameter and the numerical aperture. Thus, an excitation over the entirediameter of the geometric fiber core is presumed. The geometric fibercore is defined by the configuration of the light conducting fiber. Theoptical core diameter contrary thereto describes the portion of thefiber core that is effectively usable for light conduction.

Thus, the optical core diameter decreases with increasing length of thelight conducting fiber. This is caused by the leakage modes describedsupra which exit from the fiber core and which limit the core diameterof the light conducting fiber that is actually suitable for lightconduction with increasing fiber length. This leakage mode attenuationis independent from the so-called fiber base attenuation which occursdue to the fiber material. The leakage mode attenuation practicallyexclusively depends from the refraction index distribution in the fibercore and generally also in the fiber cross-section.

The length dependency of the optical core diameter and of the numericalaperture caused by the leakage modes is also independent from theactually provided macro-bend of the light conducting fiber and thus alsodependent from its installation type. Fiber bends are typicallydesignated as macro-bends in which the fiber axially deviates from astraight line with a bend radius in the dm, cm, or mm range. Macro-bendlosses do not result from leakage mode conduction, but are partiallycaused by modes of higher order which are not run in the core anymorefor increasing bending so that their power portion is emitted andtherefore lost. Furthermore, macro-bends attenuate each mode that is runin the fiber because the electromagnetic field reaches into the fibercladding further in the outside. This attenuation increase is thelarger, the smaller the bending radius of the fiber and the greater thewavelength of the conducted light. The longitudinal dependency of theoptical core diameter caused by leakage modes, however, also occurs whenthe fiber is oriented completely straight.

Thus, a light conducting fiber can be provided which shows littlesensitivity relative to macro-bends. However, this introduces highleakage mode conduction into the light conducting fiber which regardlessof the installation mode of the light conducting fiber, can only beinfluenced very little.

Due to the leakage mode attenuation, the optical core diameter withincreasing fiber length reaches a substantially constant value, however,the coupling properties of the light conducting fiber are degradedthrough this damping effect and additionally depend from the length ofthe light conducting fiber.

For bend insensitive light conducting fibers, therefore, the smallestpossible change of the optical core diameter and the numerical apertureas a function of the fiber length is desirable. In order to characterizethe dependency of the optical core diameter from the fiber length, ameasuring procedure is used that is defined according to theinternational GIMMF standard. Thus, the optical core diameter isdetermined at a 2 m long fiber piece through a standard measurement andcompared with the optical core diameter at a second fiber length, forexample for 300 or 1,000 m. Typically, fiber lengths are selected inwhich the length dependency of the optical core diameter becomes clearlyapparent.

Another problem when using the BIMM fibers is that the parameters thatare important for a specification of the fiber and for determiningfiber-, splicing- and coupling-losses like e.g. the optical corediameter or its NA cannot be determined in a simple manner. The fiberparameters determined through OFL, excitation using measurement methodsconfigured for standard GIMM fibers have proven too large forapplications to BIMM fibers. This leads to misinterpretations withrespect to the splicing and coupling losses that shall be estimated andalso for compatibility considerations of the measurement values comparedto standard GIMM fibers.

Thus, it is an object of the invention to provide a GIMM lightconducting fiber which is insensitive against bending which has aconfiguration under the conditions of the occurring leakage modes whichprovides the smallest length dependency of the optical core diameterthat is possible. It shall be facilitated on the one hand side toprovide a light conducting fiber which is sufficiently insensitiveagainst macro-bends in which, however, on the other hand side theleakage mode propagation is substantially minimized. This relates inparticular to providing the best possible parameters for the opticalcore diameter and the numerical aperture of the light conducting fiberfor OFL excitations while simultaneously complying with minimumrequirements for macro-bending.

This object is achieved with a bend insensitive GIMM light conductingfiber with the features of claim 1.

The bend insensitive GIMM light conducting fiber with an optical corediameter and a numerical aperture includes a core, an inner cladding, arefraction index trench and an outer cladding. It is characterized inthat the core includes a core radius, an alpha refraction index profileand a core refraction index difference with respect to the outercladding. The refraction index trench has a trench radius and a trenchrefraction index difference with respect to the outer cladding and theouter cladding has an outer cladding radius and a refraction indexbetween 1.40 and 1.55, wherein the parameters are set so that for alight wavelength of 850 nm and a overfilled launch (OFL) the opticalcore diameter for a fiber length in a range from 2 m to 300 m is reducedby less than 5% and the numerical aperture is reduced by less than 2.5%and the bend related attenuation increase for two turns and a bendradius of 7.5 mm is less than 0.2 dB.

The fiber configuration according to the invention of the bendinsensitive GIMM light conducting fiber is thus based on a core with analpha refraction index profile and combines this core with a refractionindex trench and a cladding enveloping the core. Thus, the core has apositive refraction index difference from the surrounding cladding,wherein the refraction index trench has a lower refraction indexcompared to the cladding. The basic idea of the fiber configurationaccording to the invention is to adjust the dimensions of the core, therefraction index trench and of the cladding so that the lowest possiblelength dependency of the optical core diameter and of the numericalaperture is achieved. Thus, the curvature related attenuation increaseis lowered under the recited value.

In an advantageous embodiment of the fiber configuration, the core isenveloped by an inner cladding, wherein the inner core includes an innercladding radius and an inner cladding refraction index differencerelative to the outer cladding.

In an advantageous embodiment, the inner cladding refraction indexdifference has a value of 2*10^−3 to 3*10^−3 relative to the outercladding.

Furthermore, an inner cladding with a width B can be provided in therefraction index profile between the core and refractive index trench.The inner cladding width in one embodiment is 1 to 5 μm.

In an advantageous embodiment, the core has a geometric core diameterbetween 48 and 50 μm for a numerical core aperture of 0.18 to 0.22 and acore refraction index difference between 12*10^−3 and 17*10^−3 relativeto the outer cladding. Thus, the refraction index difference of theinner cladding relative to the outer cladding is 0, the width of therefraction index trench is between 2 and 5 μm and the depth of therefraction index trench is (−6 to −11)*10^−3.

In another embodiment, the core includes a core refraction index step,wherein the ore refraction index step has a value between (−1.0 and1.0)*10^−3.

The total fiber diameter in one embodiment is between 120 and 130 μm.

Furthermore, an outer coating can be provided that envelops the lightconducting fiber, wherein the diameter of the coating is 230 to 510 μm.

The bend insensitive GIMM light conducting fibers according to theinvention shall be subsequently described in more detail with referenceto an exemplary embodiment. FIGS. 1 through 3 are being referred to. Thesame reference numerals are being used for identical or equivalentcomponents in the drawing figures, wherein:

FIG. 1 illustrates a first exemplary refraction index profile with agradient index core and a refraction index trench;

FIG. 2 illustrates a second exemplary refraction index profile with agradient index core and with a core refraction index step, an innercladding directly joining the core and a refraction index trenchconnected thereto; and

FIG. 3 illustrates a third refraction index profile with a gradientindex core and an inner cladding with an inner cladding refraction indexdifference relative to the outer cladding.

FIG. 1 illustrates a first exemplary refraction index profile of thebend insensitive GIMM light conducting fiber. The depiction illustratesthe refraction index difference relative to a reference value as afunction of the radius of the light conducting fiber. The lightconducting fiber has a core 1 with a so-called alpha-refraction indexprofile. The general diagram of the refraction index difference isdefined for example by the formula:

${\Delta\;{n(R)}} = {{dn}\;{1 \cdot \left( {1 - \left\lbrack \frac{R}{R\; 1} \right\rbrack^{\alpha}} \right)}}$

Therein dn1 is the refraction index difference in the center of thefiber for R=0, R1 is the core radius and a is the so-called profileexponent. The diagram of the core profile defined by this formuladescribes an approximation of the actually produced profiledistribution. For further illustrations, the deviation of the actualrefraction index diagram from the ideal refraction index diagram is ofsubordinate importance, since the deviations are typically very small inmodern GIMM light conducting fibers.

The core 1 is enveloped by an inner cladding 2 and a trench 3. An outercladding 4 adjoins the trench in outward direction. The inner claddinghas a radius R2, the trench has a radius R3 and a trench width b. Therefraction index of the trench is reduced relative to the refractionindex of the outer cladding and is visible in the profile as a trenchdepth dn3.

It is apparent in the instant first embodiment that the inner cladding 2is at the same refraction index level as the outer cladding 4. Anexemplary fiber design with the instant refraction index profile has forexample the following values:

R1 ˜24 μm, R2 ˜27 μm; core-NA ˜0.20; dn1 ˜13.8*10^−3; b ˜3 μm; dn3˜−9*10^−3; the radius of the entire fiber ˜62.5 μm with a radius of asurrounding coating of approximately 122 μm.

FIG. 2 illustrates another embodiment. At the beginning of the core atR1, there is a core refraction index step 1 a with a core refractionindex step in the amount of dn4. The care refraction index step dn4refers to the outer cladding refraction index and can have positivevalues (0 to 1*10^−3) and also negative values (0 to −1*10^−3). The coreradius R1 and the core refraction index dn1 essentially correspond tothe embodiment of FIG. 1. Also for this fiber design, the core 1 isenveloped by an inner cladding 2 and a trench 3. The inner cladding 2has a radius R2, the trench radius is R2.

FIG. 3 illustrates an embodiment in which the inner cladding has a lowerrefraction index level than the outer cladding. Thus, an inner claddingrefraction index difference dn2 is provided with respect to the outercladding. Exemplary values for dn2 are in the range of 2*10^−3 toapprox. −3*10^−3. In the embodiment of FIG. 3, all other parameters canbe configured essentially like in the embodiment of FIG. 1.

The light conducting fiber according to the invention was describedbased on exemplary embodiments. Further embodiments can be derived fromthe dependent claims. Additional variations are apparent to a personskilled in the art.

The invention claimed is:
 1. A bend insensitive gradient index multimodelight conducting fiber comprising a leakage mode dependent optical corediameter that is uniform over its length and a numerical aperture thatis uniform over its length, a core, an inner cladding, a refractionindex trench and an outer cladding, wherein the core includes a coreradius R1, an alpha-refraction index profile and a core refraction indexdifference dn1 with respect to the outer cladding, wherein therefraction index trench includes a refraction index trench radius R3 anda trench refraction index difference dn3 with respect to the outercladding, wherein the outer cladding includes an outer cladding radiusR4 and a refraction index between 1.40 and 1.55, wherein for a lightwavelength of 850 nm and an overfilled launch (OFL), the optical corediameter for a fiber length in a range between 2 m and 300 m decreasesby less than 5% and the numerical aperture decreases by less than 2.5%and the curvature related attenuation increase for two turns and acurvature radius of 7.5 mm is less than 0.2 db.
 2. The light conductingfiber according to claim 1, wherein the core is enveloped by an innercladding, wherein the inner cladding includes an inner cladding radiusR2 and an inner cladding refraction index difference dn2 with respect tothe outer cladding.
 3. The light conducting fiber according to claim 2,wherein the inner cladding refraction index difference dn2 has a valueof 2*10^−3 to −3*10^−3 with respect to the outer cladding.
 4. The lightconducting fiber according to claim 2, wherein the difference of theinner cladding radius R2 and the core radius R1 is 1 to 5 μm.
 5. Thelight conducting fiber according to claim 1, wherein the core has ageometric core diameter between 48 and 50 μm for a numerical coreaperture between 0.18 and 0.22 and a core refraction index differencebetween 12*10^−3 and 17*10^−3, an inner cladding refraction indexdifference of 0, a width of the refraction index trench of 2 to 5 μm anda depth of the refraction index trench of (−6 to −11)*10^−3.
 6. Thelight conducting fiber according to claim 1 wherein the core has a corerefraction index step in the amount of dn4, wherein the core refractionindex step has a value between (−1.0 and 1.0)*10^−3.
 7. The lightconducting fiber according to claim 1, wherein a total diameter of thefiber is between 120 and 130 μm.
 8. The light conducting fiber accordingto claim 1, wherein an outer coating is provided that envelops the lightconducting fiber, wherein the diameter 15 of the coating is 230 to 510μm.